Melting ATMEGA328PB - Lightning, ADC overvoltage, AREF, or LC Filter on AVcc ?

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Melting MCUs

 

I have a off grid lighting product deployed in rural Tanzania that has been experiencing a high rate of MCUs getting fried.

 

I suspect it is being driven primarily by overvoltage on Vcc (which I am dealing with by switching from buck converter to linear regulator), but I am also starting to wonder if these MCU BBQs are being caused by strange analog front end stuff with my design.  Before I post all the details, here are the questions I'm hoping the community here can help me with:

 

  1. If I select AREF to be the internal 1.1V bandgap reference, do I fry the MCU when I apply more than 1.1V to an ADC pin?
  2. If I use an LC filter on the AVcc input (as directed in datasheet), can I fry the MCU (particularly if I'm enabling/disabling the ADC frequently -- like say 4 times per second)
  3. What do you think is the most likely cause of the failures I have been seeing reported (see pictures below).

 

I recognize this post may be viewed by others as an engineer trying to get another to do my work for me, and if you feel that way then keep in mind its for a good cause (rural lighting to the poorest villages in Tanzania).  All I'm looking for here is a yay/nay about whether you think I'm on the right track in my hunt for the strange events that are causing the magic smoke to escape from my MCU.

 

Thanks in advance!


Schematic, PCB footprint

 

ATMEGA328PB running at 8MHz (internal) with following peripherals connected:
 

  • UART communications

     

    • External pullups
    • 1k series input resistance
  • Momentary Pushbutton
    • Active 5V (direct) input
    • 10M pulldown
  • 4 Analog inputs
    • Fed from quad op amp (voltage followers with high-impedence voltage dividers driving inputs)
    • also a current sense op amp
  • Some FETs
    • Control external loads
    • One FET is high side switch that turns on/off Vcc to op amps
  • 32.768 kHz external crystal
  • 4 LEDs run off of the i/o pins of MCU
    • 15 mA each (60 mA when all are turned on)

 

 

 

My BBQ'd MCUs

 

Who doesn't love the smell of burning silicon?

 

Can you tell anything about how the MCU failed when you simply look at the corpse of one?  It definitely seems like burn marks are usually aligned with the location of Vcc or AVCC pins, more or less..

 

 

 

 

 

 

 

 

 

Failure suspects
 

  • Overvoltage on Vcc (Buck converter output spikes)

     

    • I am already addressing this in next iteration of my PCB by using a linear regulator instead of buck converter
  • Overvoltage on ADC pins
    • According to datasheet, max ADC pin voltage is Vref
    • In my application I am using internal 1.1V bandgap of MCU as AREF
    • This suggests that I am in violation if any ADC pin sees more than 1.1 V, right?
  • Overvoltage on AVcc (LC filter inductive spiking?)

    ESD via discharge through plastic pushbutton plunger into MCU pushbutton detect pin

  • Overvoltage on UART lines
    • Seems like unlikely cause because I have 1k series resistors here
    • I have tested shorting UART lines to 12 V but observe no MCU BBQs

 

Overvoltage on ADC pins

 

 

I think I may be in violation of the max allowable voltage on an ADC pin when I apply power to my op amps, because when they get their 5V rail "turned on" (via high side FET), they will probably have their outputs saturated which will result in overvoltage and/or latch-up conditions as described in this analog semiconductor app note.  This means that my op amps may be supplying 5V signals to my ADC pins.  Would that result in the MCU frying?

 

Does the Vin < Vref condition mean that if I configure Vref to be AVcc instead of 1.1 internal bandgap that I can simply switchover Vref and suddenly be in compliance with datasheets ADC ratings?

 

 

Overvoltage on op amp input pins

 

I am also in violation of the input voltage range of the op amps because when I stop supplying voltage to the op-amps +5V rail, the voltage at the non-inverting terminal of each op amp is still as high as 1 V...

 

Could this be involved in frying MCU?

 

LC Filter on AVcc

 

I have implemented a LC filter as per the suggestions of the datasheet on my AVcc supply, but now I'm wondering if that is another example of "bad datasheeting" for the ATMEGA328PB, as has been discussed elsewhere

 

Here is what the datasheet says on the subject on LC filter on AVcc.  Doesn't make it sound optional, does it?

 

 

 

 

 

 

I love the smell of burning silicon in the morning

Last Edited: Wed. Jun 3, 2020 - 02:57 PM
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The only time I have seen damage like that, especially to a chip's body, is from lightning strikes.

 

I have over-voltage'd AVRs, I have powered them reverse polarity, I have dumped excess current into IO pins. Many many times they have survived. They have never done what we see in your photos.

#1 Hardware Problem? https://www.avrfreaks.net/forum/...

#2 Hardware Problem? Read AVR042.

#3 All grounds are not created equal

#4 Have you proved your chip is running at xxMHz?

#5 "If you think you need floating point to solve the problem then you don't understand the problem. If you really do need floating point then you have a problem you do not understand."

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No you should be ok on your ADC inputs as long as the pin does not exceed VCC+0.5v regardless of the Vref you choose, I have many app where I change Vref between aVCC and 1.1v where voltages of some pins are higher then Vref,

when 1.1v is selected, ie. some are 0-5v some are 0-1.1v,  when using the 1.1v as aRef, any voltage above that on a pin will read 1023 (max value) but other wise will not harm the ADC section.

My guess is your seeing failures from your dc to dc converter malfunction or near by lightning strikes.  How well grounded is the case of your controller?

Some of the pins are labeled that you are actively disabling GND in some portion of the product, that may lead to interesting paths for external ESD (lightning) effects, you may want to investigate using a high side disconnect rather then floating GND's.

 

Interesting project, hope you find a cure.

 

Jim

 

 

(Possum Lodge oath) Quando omni flunkus, moritati.

"I thought growing old would take longer"

 

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Thanks for response. 

 

The only time I have seen damage like that, especially to a chip's body, is from lightning strikes.

 

Given that characterization, I think I'm looking for a pretty high energy event (possibly a sustained event)?

 

What about if a user connected a PV panel to the Vbat+ bus that fried the buck converter IC in a closed position which in turn effectively connects 20-25V to the +5V bus?

 

Would applying 25V to the MCU cause this sort of damage?

I love the smell of burning silicon in the morning

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jaza_tom wrote:

Would applying 25V to the MCU cause this sort of damage?

 

Be interesting to try. I might have a go in the morning; I've got some random 328's in SMT that I don't need.

#1 Hardware Problem? https://www.avrfreaks.net/forum/...

#2 Hardware Problem? Read AVR042.

#3 All grounds are not created equal

#4 Have you proved your chip is running at xxMHz?

#5 "If you think you need floating point to solve the problem then you don't understand the problem. If you really do need floating point then you have a problem you do not understand."

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ki0bk wrote:

My guess is your seeing failures from your dc to dc converter malfunction or near by lightning strikes.  How well grounded is the case of your controller?

Some of the pins are labeled that you are actively disabling GND in some portion of the product, that may lead to interesting paths for external ESD (lightning) effects, you may want to investigate using a high side disconnect rather then floating GND's.

 

The case of the controller is not grounded, it is a portable battery powered product.  The case is injection molded plastic.  Would it even be possible to "ground" a plastic enclosure?

 

That being said, while the product is being charged, the PCBA's GND net is connected to some external charger which *should* be earth-grounded.  Does the lightning strike scenario come into play here?

 

The AGND vs GND nets shown in the schematic are just labelled that was because they are isolated from ground loops from one another.  They are in fact galvanically connected directly under the "melted AVcc" spots shown in my pictures (where the analog ground plane and digital ground plane meet):
 

 

I love the smell of burning silicon in the morning

Last Edited: Mon. Jun 1, 2020 - 06:29 PM
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Solar power, there is another twist to the plot, it is not uncommon for some solar controllers to control charging by using a low side switch (FET in the ground return side) meaning the + side can float (to 22v or more) when the FET is open (PWM off cycle).  That may exceed the input rating of your regulator, switching or analog.

These types of solar controllers can be hard to provide proper grounding, as it may override(shorts) the internal switching element. 

Hopefully these hints will help you track down the source of your problems.  Any more detail would require seeing the schematic and/or picture of the controller, which you may not want to show.

 

Single point ground connects like yours is usually best, but whether that should be under the MPU I'll leave to other EE's with more analog experience them me.

 

Jim

Not sure if I should plug my solar controller (with high side switching)

but if you google my call sign you can find it.  There are others.

 

 

 

(Possum Lodge oath) Quando omni flunkus, moritati.

"I thought growing old would take longer"

 

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No, your circuit itself is not the issue...some external HIGH energy source is the culprit.

 

Note:

Why draw the opamps  nutty (pointing up??).  Show the vbatt dividers connected TO the opamps where they belong..it is one sensing circuit.

 

The fet circuit is shown reversed, in that +5V should be on the left flowing right towards the +5AFE output (the hookup is fine)

 

The opamps are buffers, not transimpedance (current to voltage) amps

 

Note the LMV opamp is input is good unly to about 4V, you could use a rail-rail input and use that extra volt.

The LMV offset voltage is a bit sad (7mv max), but perhaps tolerable.

 

What's the Vzero thingy?  Some type of zero-ref cal?  why not use software?

 

It looks like a high energy trouble, but you neglect to show the power supply, charger, or battery.

Also mosi, miso, sck go to places & energy unknown!

 

switch --- 10M pulldown

That is WAY too high, maybe 100k, even that can be noisy pickup, perefer 10-20 K

 

    • This suggests that I am in violation if any ADC pin sees more than 1.1 V, right?

 

no damage, though your readings will max out at vref level.  Don't go above Avcc.

 

Double check the physical PCB (not the layout) to verify the GND & AGND are tied together...if due to some cad mishap, you will pay the price.

 

 

Comrade, we hope to see more soon.

 

 

 

 

 

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Mon. Jun 1, 2020 - 08:13 PM
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Looks like the effects of lightning to me. Remember that current flows in a loop - you’d be amazed at the torturous path that can be followed. The plug in charger makes the avr the meat in the sandwich.
Grounding the plastic box is not the solution even if it were metallic. You need to provide a more favorable path for the energy well before it gets to the avr.

It should be simple to determine if it is lightning - when did the box fail and were their storms around that time? And we’re not talking about direct hits - if lightning hits a building, you’ll know it. Nearby strikes are more likely. There’s two mechanisms - electrostatic and electromagnetic. With electromagnetic, any wiring forms a turn in a transformer. The lightning strike is the primary turn. Think of the total circuit from solar panel to whatever. Grounded at each end. Where is the current going to flow? Expect a few thousand volts, so flashover is expected. Don’t have external circuits running across your pcb - everything should enter/exit from one side. Use movs,tvs diodes,capacitors to protect each input/output to a common low impedance point that should find its way back to earth through a thick wire. Think of it like water - create drains etc so the water flows where you want so it doesn’t flood.

Last Edited: Mon. Jun 1, 2020 - 10:34 PM
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Are there any marks on the enclosure from possibly arcing through the plastic shell? These would  probably be circular, maybe up to pencil size though probably smaller, and look like a small ring of burnt plastic.

 

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

Last Edited: Mon. Jun 1, 2020 - 10:31 PM
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jaza_tom wrote:
Would applying 25V to the MCU cause this sort of damage?

I would say definitely yes.  I'll bet if you look closely at the holes in damaged units you'll see evidence that the plastic is actually fractured and not melted.  Over voltage like that with enough current capability behind it will make these things go with a very quick pop rather than a slow fizz.

Letting the smoke out since 1978

 

 

 

 

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Wow, thanks for all the responses!  Since there seems to be some interest here, I might as well show you a few other pics too:

 

Here is one of our charging stations in the middle of a village (not usually so many people milling about, but this was the grand opening):
 

 

Here is our charger, which charges up to 12 of these batteries at a time

 

 

And here is one of the portable batteries that our customers rent from us (think propane tank swap model that we have in north america):

 

 

 

 

OK, let me get to some of your questions for me:

 

avrcandies wrote:

Note:

Why draw the opamps  nutty (pointing up??).  Show the vbatt dividers connected TO the opamps where they belong..it is one sensing circuit.

 

I'm just weird like that I guess.  Next time I'll make it easier on the eyes.

 

avrcandies wrote:
The fet circuit is shown reversed, in that +5V should be on the left flowing right towards the +5AFE output (the hookup is fine)

 

I think the circuit is shown correctly.  The idea behind this FET switch is stop supplying power from the 5V net to the +5_AFE net (the analog 5V rail that the op amps use) in order to achieve lower sleep mode power.

 

avrcandies wrote:
What's the Vzero thingy?  Some type of zero-ref cal?  why not use software?

 

The Vzero thingy is indeed a zero reference thingy.  The strategy I'm using is to have Vzero = 0 when battery is discharging, and when the current goes negative, I get the MCU to create a new 0 mA offset voltage (Vzero) to feed to the low side INA181 current sense op amp.  The MCU feeds 5V to a resistor divider (Vzero select) that is buffered by one of the LMV op-amps.  The MCU then reads back that Vzero voltage, along with the output of the INA181 current sense to determine the charging current.  I set it up this way to effectively double current measurement resolution (and because I had an "exta" op amp to use in the LMV quad package).

 

avrcandies wrote:

It looks like a high energy trouble, but you neglect to show the power supply, charger, or battery.

Also mosi, miso, sck go to places & energy unknown!

 

I was trying to be selective in not overwhelming the post with too many schematics, but since you asked, here you go! 

 

5V buck converter

 

This powers the MCU.  (Note that the buck converter design was generated from the TI WebBench design tool).  D2 clamps +5 net from going below -0.3V

 

 

 

Off-board connectors

 

Note that the ISP signals are used during manufacturing only, and have no physical access.

 

 

As far as the charger goes, the only "authorized" charger (i.e. the only charger that is supposed to be charging the battery) is a cheap AliBaba DC/DC buck/boost converter with "MPPT" setpoint (basically its a buck converter with control loops modified to support CC and CV output setpoints via potentiometers, along with a "MPPT" pot that sets the target input voltage so that output power decreases if input voltage falls below certain threshold).  It's like this one (one we use doesn't appear to be on AliBaba anymore).  The charger hooks up via the DIN connector shown in schematics, above

 

 

ki0bk wrote:
Solar power, there is another twist to the plot, it is not uncommon for some solar controllers to control charging by using a low side switch (FET in the ground return side) meaning the + side can float (to 22v or more) when the FET is open (PWM off cycle).  That may exceed the input rating of your regulator, switching or analog.

 

 

Good point!  In our setup, the only external power source  that is authorized (by customer contract) to be connected to our smart battery is our central charging system (which uses the AliBaba buck/boost convertes described above).  The only ground connection to our smart battery is via the DIN connector, which connects to the negative output of the converter.  The input of the converter is connected to an off grid 26V DC bus powred by a deep cycle battery bank.

 

Kartman wrote:
Looks like the effects of lightning to me.

Another lighting vote!  I'm starting to get afraid of lightning

Kartman wrote:
It should be simple to determine if it is lightning - when did the box fail

The failures are happening in coastal tropical locations, as shown on this lightning strike map.  Looks like lightning strikes are pretty prevalent in one of our areas of operations.  I think its safe to assume that lightning strikes will be pretty constant

If there is, say, 20 m of 22 AWG cabling acting as antennas between the smart battery's + and GND outputs, and lightning strikes nearby, things could definitely get interesting in terms of electromagnetic transient coupling

 

 

 

Kartman wrote:
Use movs,tvs diodes,capacitors to protect each input/output to a common low impedance point that should find its way back to earth through a thick wire.

 

 

 

In next PCBA design iteration, I plan on having both the charger and DC loads on the same external bus, and interface with it using a Bi-Directional high side switch. 

 

This new configuration will have a strong  "23.5 V power zener" protecting the External  net from overvoltage transients (30 mΩ, 2 W continuous power N-channel FET controlled by 21V zener).  I'm guessing that arrangement should help reduce effects of lightning strikes (if the electromagnetic coupling is causing positive voltage spike), but is probably not enough, particularly for "negative going" transients.

 

Here's the design for that that I've been playing around with

 

Any way to estimate how much coupled electromagnetic energy I would need to dissipate in a worst case 40 m of 22 awg wire acting as an atenna near (say 100 m) of the strike?

 

I'm hoping I could just use a cheap 28 V MOV with 20A peak surge and 0.05 J energy rating

 

ka7ehk wrote:
Are there any marks on the enclosure from possibly arcing through the plastic shell? These would  probably be circular, maybe up to pencil size though probably smaller, and look like a small ring of burnt plastic.

 

I'm not sure, I'm doing remote debugging on this one.  Why do you ask?

 

 

 

 

I love the smell of burning silicon in the morning

Last Edited: Tue. Jun 2, 2020 - 06:11 PM
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Looks like a cool project

 

So you are showing the battery pack, not the wall charger 

I think the circuit is shown correctly.  The idea behind this FET switch is stop supplying power from the 5V net to the +5_AFE net

As mentioned: So why is it drawn backwards? The fet circuit is shown reversed, in that +5V should be on the left flowing right towards the +5AFE output (the hookup is fine)

 this like sentences write usually you do

 

Where are all your fuses and/or polyswitches?  You certainly don't have a high power battery without these??   Each jack needs a short circuit protection as a minimum.

The battery is generally an input to your "battery box" (unless charging) and should be on the left portion of the schematic, facing right.

 

Where is lights_gnd...needs to connect to something.

..this is why I somewhat despise schematics that appear as chopped up mixed salad.  If I open a road atlas, I expect to see a road atlas, bits of paper all over showing 500 feet of road.   

 

Note those four JACK1....JACK4 are not inputs...they supply power as OUTPUTS, and should be facing RIGHT & also preferably be on the right side of the schematic.

 Don't feel bad, I say this often!

 

What is  D2 bat54 for?  The only thing it does is keeps +5v from going below gnd....whyy have it?

 

You say nothing about the battery specs...why?

 

Your big wall charger is perhaps putting out too much voltage & frying your boards...the TPS 5V part is only rated to take 17V...so perhaps the charger is guilty.  Where id the charger from? 

 

this appears to go nowhere--why:

 

 

 

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Tue. Jun 2, 2020 - 08:19 PM
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Looking at the picture of the charging station - imagine you are a transient - what is the easiest way back to earth?
My guess is anything you do in the battery box is probably going to be ineffective. Proper earthing of the solar panel and charger would be my first line of investigation. There should be copper bar from the solar panel to earth along with a copper strap from the charger to this bar. Installed correctly this should shunt most of the lightning energy away from your batteries. By copper bar i mean 1” wide by 3mm thick (how’s that for mixing units!). If copper is too expensive, the aluminum will do. There should be standards to guide the precise method of earthing. As for expected energies, there”s plenty of studies that can be Googled. From memory something like a 20us pulse of 10,000A is the average lightning strike. Even 1% of that is significant energy to do damage. With the pulse and current involved, even small amounts of inductance will cause significant voltage rise. Like a karate master, deflect the energy rather than oppose it.
As for your battery packs, an internal metalic shield might be advisable. No use spraying a conductive paint as that will just vaporise on the first hit. Something like 1.2mm aluminum box. Connect the 0V of your circuit to this. A varistor on the input should clean up the rest of the nasties.
As i mentioned before, find out more about when and where the problems occur - a picture hopefully will emerge.

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 Proper earthing of the solar panel and charger would be my first line of investigation

Except, there is no solar.  charging is done by this octopus wall charger (details unknown):

    Here is our charger, which charges up to 12 of these batteries at a time.... the only external power source  that is authorized (by customer contract) to be connected to our smart battery is our central charging system.

 

Doesn't exactly seem like it would be lightning if these were burned out all at individual times (that would be very unlucky chance)...If they all burned out in 1 or 2 batches, then perhaps lightning took out a handful each swipe.  And there would prob be wires melted & a variety of defects.

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Wed. Jun 3, 2020 - 02:32 AM
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avrcandies wrote:

 Proper earthing of the solar panel and charger would be my first line of investigation

Except, there is no solar.

 

So... what is this?

Ross McKenzie ValuSoft Melbourne Australia

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So... what is this?

Perhaps...nowhere does he bring up solar at all ...but hmmmm...is that an inverter on the wall?   Strange he would not mention solar or at least say he is using it.     

only mentions

Here is our charger, which charges up to 12 of these batteries at a time.   " our central charging system. "

 

If you zoom in you can barely make out MPPT...def a solar setup!

You win a battery pack & 3 spare fuses!

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Wed. Jun 3, 2020 - 05:36 AM
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His post #4 mentions a "PV panel"...

Ross McKenzie ValuSoft Melbourne Australia

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avrcandies wrote:
..this is why I somewhat despise schematics that appear as chopped up mixed salad.  If I open a road atlas, I expect to see a road atlas, bits of paper all over showing 500 feet of road.   

 

See attached PDF

 

avrcandies wrote:

Where are all your fuses and/or polyswitches?  You certainly don't have a high power battery without these??   Each jack needs a short circuit protection as a minimum.

 

Our battery is 200 Wh 11.7 V Li-Ion with integrated protection circuits for cell balancing/charging protection/discharge protection/short circuit protection etc.

 

After the power leaves the battery's protected outputs, our PCBAs' over current protection is provided for all jacks as a block to 2A via the low side automotive load switch FET_LIGHTS with integrated overcurrent/thermal protection which limits current to approx 3A, until it heats up enough and then it cycles on/off due to thermal shutdown/hysteresis.  The short circuit shutdown comes into play when the MCU detects that low-pass filtered battery current is > 1.75A and shuts off the lighting FET within max 1 second.

 

There is also overcurrent redundancy provided by the Li-Ion battery with built in protection module that latches itself off in extended high load current scenarios. (20 Amp).

 

It may not meet UL standards XYZ, but we are operating in a market that has virtually no regulations for these sorts of things (let alone home insurance), and we retain ownership of the smart batteries throughout their life cycle, and thus we have not bumped up against any standards requirements yet, other than the rudimentry PVOC testing done before the product ships from China.

 

Kartman wrote:
Nearby strikes are more likely. There’s two mechanisms - electrostatic and electromagnetic. With electromagnetic, any wiring forms a turn in a transformer. The lightning strike is the primary turn. Think of the total circuit from solar panel to whatever. Grounded at each end. Where is the current going to flow?

Kartman wrote:
Looking at the picture of the charging station - imagine you are a transient - what is the easiest way back to earth?
My guess is anything you do in the battery box is probably going to be ineffective. Proper earthing of the solar panel and charger would be my first line of investigation. There should be copper bar from the solar panel to earth along with a copper strap from the charger to this bar. Installed correctly this should shunt most of the lightning energy away from your batteries. By copper bar i mean 1” wide by 3mm thick (how’s that for mixing units!). If copper is too expensive, the aluminum will do. There should be standards to guide the precise method of earthing.

 

avrcandies wrote:
Doesn't exactly seem like it would be lightning if these were burned out all at individual times (that would be very unlucky chance)...If they all burned out in 1 or 2 batches, then perhaps lightning took out a handful each swipe.  And there would prob be wires melted & a variety of defects.

 

Thanks for the info.  Right now we are earth-grounding the charging stations through big straps as you suggest.  This grounds the system's DC(-) bus (MPPT negative output, and battery bank negative) as well as the metal roof/PV module racking/frames

 

We have not seen any evidence of electrostatic lightning effects damaging our batteries (blown MPPT/chargers, multiple smart batteries that were charging getting fried in a single swipe as you say). 

 

I think that if lightning effects are causing these issues that it is more likely though electromagnetic coupling to the lightning's EMP while the end user has many meters of lighting cable attached to the smart battery (acting like an antenna) during a lightning storm.

 

Kartman wrote:
As i mentioned before, find out more about when and where the problems occur - a picture hopefully will emerge.

 

The when/where problems occur analysis seems to show that there is a correlation between rainy season and smart batteries getting destroyed, which lends creedence to the lightning damage theory.  The problem with determine when/where the problems occur on a higher resolution level than this is that this data can only be collected via word of mouth and estimates (customer says "this battery I rented 20 days ago stopped working about 2 weeks ago"), and that data is not being collected in a way that allows temporal analysis.

 

 

Kartman wrote:
As for expected energies, there”s plenty of studies that can be Googled. From memory something like a 20us pulse of 10,000A is the average lightning strike. Even 1% of that is significant energy to do damage. With the pulse and current involved, even small amounts of inductance will cause significant voltage rise. Like a karate master, deflect the energy rather than oppose it.

 

You are talking about electromagnetic coupling between two galvanically isolated systems (i.e. the sky/earth system  on one hand and the smart battery's loads/external wiring on the other hand) right?  

 

I tried Googling for studies about lightning electromagnetic pulse (LEMP) coupling transients in embedded systems due to lightning strikes, but all I could find were studies on the effects of LEMP on power utilities' distribution/transmission lines for the most part (and they suggest similarly 10-20 kA for 10 µs for typical lightning strike) but when it came to estimating LEMP coupling things get foggy.  Unfortunately, almost all the studies seem to be behind paywalls...

 

I'm only concerned about lightning strikes further away than 50 m (closer than that I'm going to call "improbable").  Not sure how to estimate the induced current (at 25V MOV clamp value) to size the proper MOV.  I think that surge current and energy are the primary selection factor for the MOV (other than operating voltages) right?  To get there I need to get a rough estimate of the inductance of my lighting cables w.r.t. the lightning bolt, assume some value of coupling coefficient etc etc.  All very academic sounding.  Maybe I just go for a 500A bad boy and call it a day?

 

Seems like the range of options in my price range MOV-wise is:

* $0.11 in qty 1000 --> 30A not-so-bad-boy that can dissipate 0.3J

* $0.17 in qty 1000 --> 500A bad boy that can dissipate 1.6J

 

Maybe I manufacture half the circuit boards with one type and half with the other and run a little A/B in-service field testing to see if the not-so-bad-boy is "good enough".

 

Kartman wrote:
As for your battery packs, an internal metalic shield might be advisable.

 

This would only limit the LEMP coupling to conductors inside the enclosure, but it seems to me that the much bigger coupling values will occur on the many meters of light cables strung up around a customer's house.  So, even if the i had an aluminum enclosure that my PCBA 0V is grounded to via beefy cable, that won't help with the big external antenna (the user's wiring for lights) that is being connected to the PCBA right?

 

 

 

 

Attachment(s): 

I love the smell of burning silicon in the morning

Last Edited: Wed. Jun 3, 2020 - 03:08 PM
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His post #4 mentions a "PV panel"...

True, however he is not saying there IS solar in their system & he is talking about the battery pack user, not the design of the system.

 

What about if a user connected a PV panel to the Vbat+ bus that fried the ​​​​​.....​​

 

Why not state that the system itself is solar powered

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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avrcandies wrote:

His post #4 mentions a "PV panel"...

True, however he is not saying there IS solar in their system & he is talking about the battery pack user, not the design of the system.

 

What about if a user connected a PV panel to the Vbat+ bus that fried the ​​​​​.....​​

 

Why not state that the system itself is solar powered

 

Didn't really seem relevant at the time since the failures seem to be happening while the end user has the smart battery rented.  In other words, as far as I know, there *should* be no external power sources connected to the smart battery at the time of failure.

I love the smell of burning silicon in the morning

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How good a ground/earthing connection do you have ? (if everything on same batt. have the same ground, like a normal house installation it should not be a big problem) 

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The power section looks like a crazy mess?  Grounds thrown randomly around all over the place?   Why are grounds being disconnected?   Lots of bad things can happen when grounds are suddenly disconnected.  For example a voltage divider no longer divides & applies high voltage.  Apparently the connector, Jack3, is not really tied to a ground at all but some other component shown elsewhere. Draw it neatly & cohesively & defects will be much more noticeable.  

This is exactly how problems get overlooked.  I'm just stressing this, since a clear schematic roadmap is vital to understanding what is going on, especially when things are mysteriously failing.  Walk the schematic in one direction left to right, gnd always points down, etc.

 

Note this fet won't work, it cannnot block the charger+  due to the internal diode.  

 

 

 

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Wed. Jun 3, 2020 - 03:48 PM
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avrcandies wrote:

The power section looks like a crazy mess?  Grounds thrown randomly around all over the place?   Why are grounds being disconnected? 

 

I had chose low side switches due to their superior efficiency (low Rds on) for a given price point.  Our product has to be extremely inexpensive to build for the unit economics to make sense.

 

avrcandies wrote:

Apparently the connector, Jack3, is not really tied to a ground at all but some other component shown elsewhere.

 

 

avrcandies wrote:

I'm just stressing this, since a clear schematic roadmap is vital to understanding what is going on, especially when things are mysteriously failing.  Walk the schematic in one direction left to right, gnd always points down, etc.

avrcandies wrote:

Note this fet won't work, it cannnot block the charger+  due to the internal diode.  

 

That FET is just there to protect against reverse polarity charger connections.

 

I love the smell of burning silicon in the morning

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I’d suggest a design review is done for the electronics. Why is there a crystal if the product is cost constrained? If the electronics are only doing charge and load protection, then precise timing is not required methinks.
Switching the low side of the load might not be the best idea considering the application. Apart from lightning, have you tested for short circuits?

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That FET is just there to protect against reverse polarity charger connections.

That's fine, however, the diode will allow charger current to flow through the fet regardless of whether charge_disable is high or low.

 

 

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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The opamps are doing very little for the stated goal - the 100nF caps alone will achieve the < 10k impedance requirement for the adc.
Why did you not go for high side current sense? And high side switching of the load?
Your earlier question regarding sizing of the protection components - that’s always a tricky one. There’s been plenty of published studies regarding the energies involved. Nevertheless, experience counts as every situation is different. I’d start with a 14mm varistor and see how that works in the field.
You might want to investigate how the lighting and other loads are connected to your box. What is the length of wire? Is it coiled? And so on. Every bit of information counts.

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The opamps are doing very little for the stated goal - the 100nF caps alone will achieve the < 10k impedance requirement for the adc.

You do have to watch out for bias currents at the AVR pin, which can effect the readings through high ohms, so check it when using hundreds of kohms (or lower if you really need all 1000 counts).  In those cases, opamp can be good, but then watch out for its offset & noise.

 

The schematic says they are transimpedance amps, however that is not correct & still needs updated-- not sure the OP is keeping up with all the needed changes.

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!